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Homework Help: Thermodynamics Hw help, piston designing

  1. Nov 17, 2012 #1
    1. The problem statement, all variables and given/known data
    Have a design project in thermodynamics. The problem reads,

    Someone has suggested that the device shown in the figure below be used to improve the maximum force F against the spring, which has a spring constant of k. This is accomplished by changing the temperature of the liquid-vapor mixture in the container. You are to design such a device to close sun-blocking window-shutters that require a maximum force of 0.5 lbf. The piston must move 6 inches to close those shutters completely. You elect to use R-134a as the working fluid and arrange the liquid-vapor mixture container such that he temperature changes from 70 degrees F when shaded from the sun to 100 degrees F when exposed to the full sun. Select the sizes of the various components in this system to do this task. Also select the necessary spring constant and the amount of R-134a to be used.

    2. Relevant equations


    3. The attempt at a solution
    All of my work and all the info I have been using is in the attachments. I have been working it on a whiteboard at school. I can only load 3 pics. I continued on and found that the natural length of the spring is 9.707 inches with a k value of 0.031833 Lbf/in.

    I am assuming that at 100 degrees F that the system is in equilibrium and the force to keep the windows closed is 0.5 lbf. The picture also shows the piston is closed off, so I am assuming that I don't need to worry about atmospheric pressure. I can't see where I am going wrong, but the piston diameter seems to small. Is this right or am I reading the original problem wrong.

    Attached Files:

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    • 2.JPG
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    • 3.JPG
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  2. jcsd
  3. Nov 19, 2012 #2
    Got it figured out.
  4. May 21, 2013 #3
    I have assignment as what you did. So can you please give me your answer? so I can get some ideas. Please Help me.
  5. Nov 25, 2013 #4
    Hi Nilkc, i have the same project and i'm having trouble with that, could you give me a hint?? hELP
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